Absorption methods using an aqueous amine solution are commonly used for removing acid compounds (notably CO2, H2S, COS, CS2, SO2 and mercaptans) present in a gas. The gas is deacidized by contacting with the absorbent solution, then the absorbent solution is thermally regenerated. For example, document U.S. Pat. No. 6,852,144 describes a method of removing acid compounds from hydrocarbons. The method uses a water-N-methyldiethanolamine or water-triethanolamine absorbent solution containing a high proportion of a compound belonging to the following group: piperazine and/or methylpiperazine and/or morpholine.
One limitation of the absorbent solutions commonly used in deacidizing applications is their insufficient H2S absorption selectivity in relation to CO2. Indeed, in some natural gas deacidizing cases, selective H2S removal is sought by limiting to the maximum CO2 absorption. This constraint is particularly important for gases to be treated with a CO2 content that is already less than or equal to the desired specification. A maximum H2S absorption capacity is then sought with maximum H2S absorption selectivity in relation to CO2. This selectivity allows to maximize the amount of treated gas produced and to recover an acid gas at the regenerator outlet having the highest H2S concentration possible, which limits the size of the sulfur chain units downstream from the treatment and guarantees better operation. In some cases, an H2S enrichment unit is necessary for concentrating the acid gas in H2S. In this case, the most selective amine is also sought. Tertiary amines such as N-methyldiethanolamine or hindered amines exhibiting slow reaction kinetics with CO2 are commonly used, but their selectivities are limited to high H2S feed ratios.
Another limitation of the absorbent solutions commonly used in total deacidizing applications is too slow CO2 or COS capture kinetics. In cases where the desired CO2 or COS specifications level is very high, the fastest possible reaction kinetics is sought so as to reduce the height of the absorption column, this equipment under pressure, typically between 20 bars and 90 bars, representing a significant part of the investment costs of the process.
Whether seeking maximum CO2 and COS capture kinetics in a total deacidizing application, or minimum CO2 capture kinetics in a selective application, it is always desirable to use an absorbent solution having the highest cyclic capacity possible. This cyclic capacity, denoted by Δα, corresponds to the feed ratio difference (a designates the number of moles of absorbed acid compounds nacid gas per kilogram of absorbent solution) between the absorbent solution fed to the absorption column and the absorbent solution discharged from the bottom of said column. Indeed, the higher the cyclic capacity of the absorbent solution, the more limited the absorbent solution flow rate required for deacidizing the gas to be treated. In gas treatment methods, reduction of the absorbent solution flow rate also has a great impact on the reduction of investments, notably as regards absorption column sizing.
Another essential aspect of gas or industrial fumes treatment operations using an absorbent solution remains the regeneration of the separation agent. Regeneration through expansion and/or distillation and/or entrainment by a vaporized gas referred to as “stripping gas” is generally considered depending on the absorption type (physical and/or chemical).
Notably, a limitation of the absorbent solutions commonly used today is the energy consumption necessary for solvent regeneration that is too high. This is particularly true in cases where the acid gas partial pressure is low. For example, for a 30 wt. % 2-aminoethanol (or monoethanolamine or ethanolamine or MEA) aqueous solution used for post-combustion CO2 capture in thermal power plant fumes, where the CO2 partial pressure is of the order of 0.12 bar, the regeneration energy represents approximately 3.7 GJ per ton of CO2 captured. Such an energy consumption represents a significant operating cost for the CO2 capture process.
It is well known to the person skilled in the art that the energy required for regeneration by distillation of an amine solution can be divided into three different items: the energy required for heating the absorbent solution between the top and the bottom of the regenerator, the energy required for lowering the acid gas partial pressure in the regenerator by vaporization of a stripping gas, and the energy required for breaking the chemical bond between the amine and the CO2.
These first two items are inversely proportional to the absorbent solution flows to be circulated in the plant in order to achieve a given specification. In order to decrease the energy consumption linked with the regeneration of the solvent, the cyclic capacity of the solvent is therefore once again preferably maximized.
The last item relates to the energy required for breaking the bond created between the amine used and the CO2. To decrease the energy consumption linked with the regeneration of the absorbent solution, the binding enthalpy ΔH is thus preferably minimized. However, it is not easy to find a solvent with a high cyclic capacity and a low reaction enthalpy. The best absorbent solution from an energy point of view is therefore the one allowing to reach the best compromise between a high cyclic capacity Δα and a low binding enthalpy ΔH.
The chemical stability of the absorbent solution is also an essential issue in gas deacidizing and treatment processes. Degradation resistance is a limitation for the commonly used absorbent solutions, notably under regeneration conditions at temperatures ranging between 160° C. and 180° C. considered in CO2 capture processes. These conditions would allow the CO2 to be recovered at a pressure ranging between 5 and 10 bars, thus enabling to save energy on the compression of the CO2 captured with a view to the transport and storage thereof.
It is thus difficult to find compounds or a family of compounds allowing the various deacidizing processes to operate at lower operating costs (including the regeneration energy and the costs related to losses due to degradation) and investment costs (including the cost of the absorption column), in terms of post-combustion CO2 capture as well as gas treatment deacidizing.
It is well known to the person skilled in the art that tertiary amines or secondary amines with severe steric hindrance, as described in the articles by G. Sartori et al. published in Industrial Engineering and Chemistry Fundamentals, 22, (1983), 239-249, and Separation and Purification Methods, 16 (2) (1987), 171-200, have slower CO2 capture kinetics than little-hindered primary or secondary amines. On the other hand, tertiary or secondary amines with severe steric hindrance have instantaneous H2S capture kinetics, which allows selective H2S removal based on distinct kinetic performances.
Among the applications of these tertiary or hindered amines, U.S. Pat. No. 4,405,582 describes a method of selective H2S removal from gases containing H2S and CO2 by means of an absorbent containing amines of diaminoether type where at least one of the two amine functions is tertiary.
Patent JP-8,257,353 describes a method of capturing CO2 in combustion fumes, based on amine solutions corresponding to the general formula:

The general formula of the diamine provided in this patent exclusively imposes as the alkyl ether unit joining the two nitrogen atoms an ethoxy ethyl-CH2CH2—O—CH2—CH2-unit. More particularly, the compound of interest is bis(2-dimethylaminoethyl)ether in contact with combustion fumes. However, this document does not describe the degradation resistance performances of this molecule.
The inventors have discovered that amines of diaminoether type are not equivalent in terms of performance for use in absorbent solution formulations for acid gas treatment in an industrial process. Some molecules of diaminoether type have insufficient performances, notably as regards the chemical stability thereof, for suitable use in acid gas treatment. A contrario, other molecules exhibit particularly high chemical stability and absorption performances.
The object of the present invention is the use of particular molecules belonging to the bis(aminoalkyl)ether family exhibiting optimum performances for CO2 capture capacity, selective H2S removal and thermal stability within the context of gas deacidizing. These bis(aminoalkyl)ethers exhibit the specific feature of having at least one dialkyl aminopropyl ether unit. More precisely, these molecules are bis(dialkylamino-3-propyl)ethers with the general formula as follows:
and (dialkylamino-2-ethyl)-(dialkylamino-3-propyl)ethers with the general formula:

In both general formulas (I) and (II), each radical R is independently selected from among a methyl radical or an ethyl radical.